1. Field of the Invention
The present invention relates to a semiconductor memory device and a method for fabricating the same, and more particularly, to a contact structure for a semiconductor memory device that reduces contact defects and contact resistance and a method for forming the same.
2. Description of the Related Art
In a semiconductor memory device such as dynamic random access memory (DRAM) devices having a plurality of memory cells each consisting of one access transistor and one storage capacitor, an increase in cell density inevitably demands a reduction in the size of the transistor and the capacitor in order to prevent an increase in memory chip size. The higher the integration density, the smaller the size of the semiconductor device. Therefore, there is a demand for an advanced technique for forming a contact between conductive layers in order to ensure operation of the semiconductor memory device. A reduction in the size of the semiconductor device leads to a reduction in operating voltage. For instance, a power supply voltage of 1.2 V has been proposed for a 256-Megabit DRAM whereas a power supply voltage of 1 V or less has been proposed for a 1-Gigabit DRAM.
Accordingly, a need exists for an advanced contact forming technique capable of reducing the semiconductor device in size and providing a good ohmic contact at lower power supply voltages. In order to contact a lower conductive layer having a limited area with an upper conductive layer through an insulation layer formed on the lower conductive layer, a technique for reducing a contact resistance by forming at least two through openings has been proposed. However, the space between the adjacent through openings is narrow due to the reduction of the semiconductor device. If the through openings are overlapped, a peak point P may be generated as shown in FIG. 1.
Referring to FIG. 1, openings C1 and C2 passing through an interlayer insulation layer 30 are formed on a lower conductive layer 20 on a substrate 10. The through opening C1 has an etched surface 31 formed by isotropic etching at an upper side and has an etched surface 33 formed by anisotropic etching at a lower side. Similarly, the opening C2 has an etched surface 32 formed by isotropic etching at an upper side and has an etched surface 34 formed by anisotropic etching at a lower side. Thus, an isotropic etching of the insulation layer 30 is necessary to form the surfaces 31 and 32. The peak point P is produced by excessive anisotropic etching of the insulation layer 30. If a contact is made by filling the openings C1 and C2 with a metal contact plug, contact related defects, for example, poor contact, migration, and the like, can occur during subsequent processes. Moreover, since the lower contact area of the contact plug is considerably smaller in size than the upper contact area, a contact resistance problem arises.
To overcome these problems in a double contact structure such as that shown in FIG. 1, a photo masking process for forming the contact has been proposed as indicated in FIG. 2. This technique has a scheme for forming one through opening having the size corresponding to the total size of the adjacent through openings on an interlayer insulation layer. In detail, a mask 50 having a rectangular pattern 52 formed on a glass substrate 51 coated with chromium is provided to form a pattern on a photoresist 40. The size of the rectangular pattern 52 is determined by considering the total size of the two through openings. In an exposing process, a light beam such as a laser beam or ultraviolet ray pass through the pattern 52 of the mask 50, reaching the upper side of the photoresist 40. Further, the light beam is reflected at the surface of the glass substrate 51, not passing through it. Therefore, an exposed part 41 is distinguished from an unexposed part 42. Only subsequent processes develop the exposed part 41.
If the exposed part 41 is etched after development, one contact hole is formed as the through opening on an insulation layer 30 of an oxide layer. If the remaining unexposed part 42 of the photoresist 40 is removed and the through opening is filled with tungsten, there is formed the contact plug for contacting a partial surface of a lower conductive layer 20 with an upper conductive layer to be formed on the insulation layer 30. However, since the plane of the contact plug appears as an elliptical shape instead of circular in shape, the contact area is reduced. In other words, the size of the opening is smaller than the total size of the two openings. This is because the unexposed part 41 is formed as the elliptical shape due to a diffraction of light and an interference phenomenon during an exposing process. Consequently, the contact area is reduced by the pattern of the rectangular shape, and it is difficult to provide a good ohmic contact at lower power supply voltages. Hence, it is necessary to both increase the contact area and reduce the contact related defects.